Application of an Equiaxed Grain Growth and Transport Model to Study Macrosegregation in a DC Casting Experiment
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TRODUCTION
ONE of the main phenomena causing macrosegregation in direct-chill (DC) cast aluminium alloy products is the motion of free-floating equiaxed grains. The solute-lean grains settle toward the bottom of the solidification zone (mushy zone) and eject solute-rich liquid. This tends to cause negative macrosegregation in the vicinity of the centerline of the casting, and positive macrosegregation elsewhere. The intensity of this effect on macrosegregation depends largely on the morphology of the equiaxed grains—dendritic or globular.[1–4] The grain morphology has an impact on the motion and the packing of free-floating equiaxed grains[5] as well as on the permeability of the packed grain layer. The development of the microstructure (grain size, morphology) is also strongly conditioned by macroscopic flow, which determines the transport of grains and of inoculant particles that act as nucleation sites, throughout the casting.[6] Control of microstructure and of macrosegregation in DC casting via various process parameters, such as casting speed, grain refinement or melt feeding, require a
AKASH PAKANATI and KNUT OMDAL TVEITO are with the Department of Materials Technology, NTNU, 7491 Trondheim, Norway. MOHAMMED M’HAMDI is with the Department of Materials Technology, NTNU and also with the SINTEF Materials and Chemistry, 0314 Oslo, Norway. Contact e-mail: [email protected] HERVE´ COMBEAU and MIHA ZALOZˇNIK are with the Universite´ de Lorraine, CNRS Institut Jean Lamour – IJL, 54000, Nancy, France. Manuscript submitted July 9, 2018. Article published online February 13, 2019 METALLURGICAL AND MATERIALS TRANSACTIONS A
detailed understanding of the involved physical mechanisms. Such understanding is still lacking and the support of modeling is vital to improve the control of the complex casting process. Volume-average multiscale and multiphase models that couple the description of the process-scale transport with detailed models of dendritic grain growth have been developed for around 30 years[3,7–13] and have been applied to other casting processes, mainly to steel ingot castings.[14–17] This type of modeling is much less developed for DC casting of aluminum alloys. Most DC casting models were based on simple solidification models and did not account for the description of nucleation and of grain growth kinetics.[18–23] Models that included grain growth kinetics did either not consider grain motion at all[1] or assumed a globular morphology for the free-floating grains.[23,24] Detailed models that focused on grain structure and morphology were not coupled with grain motion and macrosegregation.[25] Simulations of macrosegregation were thus limited to globular grains and could not correctly describe the influence of grain morphology. Only recently a multiscale model of DC casting, sufficiently advanced to describe the morphology development during grain growth, fully coupled with grain motion, and macrosegregation, was developed and applied by Tveito et al.[3] This three-phase volume-averaged[8] model accounts for so
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